A variety of metals having significant industrial uses are not found naturally in their elemental forms. Similarly, a variety of industrially useful metal alloys require costly metal recovery and alloying processes before the alloys can be made available for use. Typically, metals that are not available in their elemental forms are mined as a variety of compounds from which the desirable metal product must be extracted. An example of one such metal is aluminum. Commercially, aluminum is produced from naturally occurring aluminum compounds by the electrolytic reduction of Al.sub.2 O.sub.3. Al.sub.2 O.sub.3 is obtained from bauxite ore by the Bayer process which involves digesting crushed bauxite ore in a strong caustic soda solution. In 1886, electrolytic production of aluminum was invented by Charles Hall in the United States and by Paul Heroult in France, each independent of the other. This process, known today as the Hall Heroult process, transformed aluminum from a precious metal into a common structural material. The process is still today the only commercial process for obtaining aluminum metal and is fundamentally the same as it was originally disclosed by Hall and Heroult in 1886. The Hall-Heroult process, cited herein as an exemplary electrolytic metal-production process, relies upon the passage of an electric current through a molten electrolyte containing Al.sub.2 O.sub.3. An important feature of the Hall-Heroult discovery was that cryolite, a double salt of aluminum and sodium represented by the chemical formula, Na.sub.3 AlF.sub.6, would dissolve Al.sub.2 O.sub.3 and that the dissolved Al.sub.2 O.sub.3 could be electrolytically reduced to molten aluminum metal.
The electrolytic reduction of metals is often performed in large cells or pots. Hall Heroult cells, for example, have massive carbon cathodes on the bottom of the cell and carbon anodes, normally formed in the shape of large blocks, suspended above the cell and capable of being lowered into the electrolyte. Direct electric current is passed from the anode through the electrolyte to the carbon cathodes. During the reduction of Al.sub.2 O.sub.3, for example, the carbon anodes are consumed in the chemical reaction occurring in the cell. This reaction can be represented as follows: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2
This process yields an aluminum product that is very pure, e.g., 99.0% to 99.8%. The main impurities remaining in the product are traces of iron and silicon
In addition to aluminum, fused salt electrolytic processes are currently used to produce other metals including magnesium, sodium, other alkali and alkaline earth metals, and also titanium and rare earth metals Typically, magnesium is produced using either the I. G. Farben process in which the cell feed is anhydrous MgCl.sub.2 or the Dow seawater process in which the cell feed is MgCl.sub.2.1.7H.sub.2 O. The cells are made of steel and may be lined with refractory brick. The cathodes are typically steel and the anodes are typically graphite. During cell operation, molten magnesium is electrochemically reduced at the cathode surface from which it detaches and rises to the surface of the electrolyte. Simultaneously with this, chlorine gas evolves at the anode. In the Dow cell, the removal of water from the cell feed results in anode consumption.
The production of sodium is carried out at approximately 580.degree. C. by the electrolysis of NaCl-CaCl.sub.2 in the Downs cell, a cell having concentrically arranged graphite anodes and steel cathodes separated by a steel diaphragm. In such a cell, both the molten sodium metal and chlorine gas rise to their respective compartments at the top of the cell.
In the Dow-Howmet process for the production of titanium, TiCl.sub.2 is electrolyzed from a KCl-LiCl melt at a temperature of approximately 520.degree. C. The cell typically includes an anode, a deposition cathode and a feed cathode. Generally, the anode is formed of graphite and is surrounded by a diaphragm of reinforced screen coated with either cobalt or nickel. The cathodes are typically formed from steel cylinders and the feed cathode assembly is surrounded by a screen basket with which it makes electrical contact. TiCl.sub.4 is added to the feed cathode compartment where it is electrolytically reduced to form TiCl.sub.2. At the deposition cathode, the TiCl.sub.2 is reduced to titanium metal. An anode gas comprising chlorine and TiCl.sub.4 is produced, the latter component being recovered, purified and recycled back as cell feed.
One significant disadvantage of electrolytic processes that use carbon electrodes is the effect of carbon processing and use on the environment. Typically, carbon anodes must comprise high purity carbon which is fabricated by pouring into molds such carbonaceous feedstocks as coal tar and pitch followed by baking at high temperatures for extended periods of time. This process, referred to industrially as a pre bake causes undesirable by products, such as PNA's, PCB's and SO.sub.2, to be evolved from the mold. Furthermore, the problem continues during use of the anodes in the electrolysis cell proper and even in subsequent processing of cell products.
In a conventional Hall Heroult cell using pre baked consumable carbon anodes, approximately one half pound of carbon is consumed for every pound of aluminum produced. The consumed carbon issues from the cell as CO and CO.sub.2, so-called "greenhouse gases", which are suspected of contributing to long-term global warming. Additionally, the use of carbon anodes tends to result in the generation of significant amounts of carbon dust which can cause health problems for exposed workers. To contend with these problems, industry is forced to resort to complicated filtering, removal and isolation techniques which can reduce process efficiency while increasing capital and operating costs. Carbon consumption is high also in non electrochemical metals production. For example, conventional steel production methods also require the use of a form of carbon called coke. Coke ovens are a further source of carbon based pollution.
In view of the current world wide demand for steel and aluminum and the emerging markets for other ore based metals such as titanium for both industrial and military applications, a need exists for a simple, electrolytic method for the reduction of ores containing such metals into their elemental metal forms. Additionally, due to their sheer volume in the world wide market, a need exists for simple processes that lead to a direct production of metallic alloys such as carbon steel and stainless steel in environmentally acceptable ways.